1. Field of the Invention
The present invention concerns a device mounting a semiconductor element, such as an IC and LSI having multi-terminal and narrow pitch electrodes, on a wiring substrate, and more in particular, it relates to the device mounting the semiconductor element by an adhesive material and a method for mounting.
2. Description of the Prior Art
Conventionally, there is a device mounting a semiconductor elements, for example, IC and LSI having multi-terminal and narrow pitch electrodes on a wiring substrate by an adhesive material. A example of the device will be explained with reference to FIGS. 1A, 1B, 1C and FIG. 2.
As shown in FIG. 1A, an insulative resin 4 for adhesion is at first applied to a wiring substrate 1. The wiring substrate 1 is made, for example, of ceramic, glass or glass epoxy resin on the surface of a basic material 2 having a substrate wiring 3. The substrate wiring 3 is made of Cr, Ar, Al, Cu, ITO and the like which is prepared by forming a substrate wiring metal by sputtering or vapor deposition and then etching the wiring substrate metal while leaving a portion of resist for forming the substrate wiring and etching the substrate wiring metal, or using a printing process. The insulative resin 4 comprises a UV-curable or thermoset type epoxy, silicone or acrylic resin.
As shown in FIG. 1B, a protruded electrode 6, for example, made of Au, Ag or Cu formed on the semiconductor element 5 by way of electric plating or the like is positionally aligned with the substrate wiring 3 and then the semiconductor element 5 is pressed by a pressing member 30 and pressed against the basic material 2 of the wiring substrate 1. The protruded electrode of the semiconductor element 5 is pressed against the wiring substrate 1, and the UV-curable or thermoset adhesive insulative resin 4 between the semiconductor element 5 and the substrate wiring 3 is driven out.
Then, in the state pressing the semiconductor element 5 by the pressing member 30, the adhesive insulative resin 4 is cured by UV-rays if the resin is the UV-curable type or cured by heat if the resin is the thermoset type, and the pressure is released and the semiconductor element 5 is secured to the wiring substrate 1.
FIG. 1C is a view illustrating a cross sectional structure of a completed semiconductor device 40 after removing the pressing member 30. The protruded electrode 6 formed under the aluminum electrode 7 (FIG. 2) and the substrate wiring 3 are connected electrically.
However, in the prior art described above, since the UV curable or thermoset adhesive insulative resin is used for the connection between the protruded electrode of the semiconductor element and the wiring substrate, the following problems are present.
That is, in a case of use under a high temperature circumstance or by using a semiconductor element of highly heat generating semiconductor element such as a power module, the adhesive insulative resin expands under the thermal effect, and if the expansion of the insulative resin is greater than the shrinkage of the insulative resin during curing of adhering the semiconductor element to the wiring substrate, a gap is formed between the protruded electrode and the substrate wiring to bring about a problem of causing electric connection failure.
As a method of overcoming such a problem, it has been proposed a method of overcoming the foregoing problem by making the coefficient of thermal expansion of the insulative resin smaller than the coefficient of thermal expansion of the protruded electrode as disclosed, for example, in Japanese Patent Publication Hei 7-52741 published on Jun. 5, 1995.
However, such a method results in a problem of causing breakage in the semiconductor element or the wiring substrate if the coefficient of thermal expansion of the insulative resin is made smaller than the coefficient of thermal expansion of the protruded electrode. This drawback is caused, for example, by the following reason. For reducing the coefficient of thermal expansion of the adhesive insulative resin, a filler such as silicone is generally filled at a high ratio to the insulative resin but this increases young's modulus (modulus of elasticity) to apply a large stress to the semiconductor element and the wiring substrate.
As disclosed in Japanese Patent Laid-Open Hei 4-82241 published on Mar. 16, 1992, it has been proposed for overcoming the foregoing problem to form a highly elastic insulative resin layer between the basic material of the substrate wiring and the substrate wiring and they are secured by the adhesive insulative resin in a state of elastically deforming the substrate wiring. The protruded electrodes of the semiconductor element are elastically secured to the wiring of the wiring substrate by the highly elastic insulative resin layer.
However, since the method proposed above utilizes the elastic deformation of the insulative resin layer for the connection between the protruded electrode of the semiconductor element and the substrate wiring, when the semiconductor element is pressed toward the wiring substrate, the wiring substrate and the insulative resin layer are deformed by a slight force to apply stresses to the wiring substrate.
Further, the heat curing of the adhesive insulative resin requires heat at a temperature higher than the glass transition temperature Tg of the basic material for the wiring substrate (the glass transition temperature Tg of the basic material for the wiring substrate is, for example, about 120.degree. C. and, accordingly, a temperature of 170 to 250.degree. C. is required for the curing). When the adhesive insulative resin is expanded under a high temperature circumstance and if the amount of expansion exceeds the amount of shrinkage, an elastic recovery of the insulative resin layer between the substrate wiring and the wiring substrate is eliminated, because the insulative layer resin is deformed not elastically but plastically even by a slight pressure. Therefore, the protruded electrode of the semiconductor element and the substrate wiring can not be brought into contact and no electric connection can be attained.